Sound transducer



March 18, 1941. w. F. STEERS SOUNb TRANSDUCER Filed Dec. 51, 1937CHROM/UM PLfl TED Patented Mar. 18, 1941 PATENT OFFICE SOUND TRANSDUCERWilliam F. Steers, Maplewood, N. J., assignor to Radio Corporation ofAmerica, a corporation of Delaware Application December 31, 1937, SerialNo. 182,665

4 Claims. (01. 179-1155) My invention relates to sound transducers, andmore particularly to electromagnetic loudspeakers of the dynamic ormoving coil type.

One object of my invention is to provide a 5 simplified and improvedcentering structure for moving coils in dynamic speakers or microphones.

Another object of my invention is to provide an inexpensive, simplifiedand improved dust filter for a sound transducer of the dynamic type forthe purpose of preventing foreign particles entering the air gap.

A still further object of my invention is to provide, in a dynamicloudspeaker, a novel combination of dust filter and centering means forthe moving voice coil.

Another object of my invention is to produce in a dynamic loudspeaker avoice coil centering means of the sliding type that is characterized bynoiseless operation and a minimum of friction.

- In accordance with the present invention, the

moving coil in a dynamic speaker is centered in the magnetic air-gap bymeans of a layer of soft nap that is secured to the moving coilstructure and adapted to slide with the structure along smooth orpolished pole faces.

This arrangement does away with the conventional flexiblecentering'spider and the accom panying assembly and adjustingdifiiculties. Further advantages reside in dustproofing, noiselessoperation, reduction in friction, simplicity, low cost, and reduction ina tendency to warping.

I am aware of other attempts to produce an arrangement broadly similarto mine. An European loudspeaker employed a centering slide of napmaterial secured directly to the inner pole piece and arranged to slidealong the moving surface of the voice coil structure, Tests of thisspeaker showed that there resulted considerable high pitched noise fromscraping action or friction between the contact surface of the movingcoil structure and the fabric. Reference is made to the following priorpatents-Decaux 1,715,057, May 28, 1929, and Vermulen et a1. 2,006,847,July 2, 1935. r

In my improved arrangement; Iovercome the above mentioned difiicultiesby securing the sliding fabric directly to the surface of the movingcoil structure and providing highly polished pole pieces for the fabricto slide along. Since the sliding contact is between the pole surfacesand the fabric, and in view of thepolished inherently poor soundconducting nature of the surfaces and the nature of the fabric,1theoperation is substantially noiseless and with negligible friction.

The basic idea of my invention is also adapted to other uses as will beset forth more clearly in the following specification and claims. Morespecific objects and uses of the invention will also become apparentupon reading the specification and claims.

My invention is illustrated in some of its embodiments in theaccompanying drawing in which Figure 1 is a view in side elevation andpartly in section of a dynamic speaker disclosed in my copendingapplication Serial No. 82,579, filed May 29, 1936, entitledElectromagnetic transducers, but embodying my improved centering meansin place of the conventional one shown in said application,

Figure 2 is a fragmentary view in side eleva- 15 tion and partly insection of a modification or another use of my invention as applied to adouble voice coil speaker disclosed in patents of Ringel 2,007,747 andOlson 2,007,748,

Figures 3 and 4 are fragmentary views, partly 20 in section, ofmodifications of my invention.

Referring to Fig. 1, the eleetro-dynamic loudspeaker comprises amagnetic field structure formed by a U-shape yoke I of magneticallypermeable material, such as iron or mild steel, 25 an inner pole formedof similar material and including a core base 3 and a separate polepiece or core end 5, also of similar material, the core 3 and end 5being fastened in place on the yoke I as by means of a bolt 1 thatextends axially 30 through a bore in the core 3 and end 5. An annularpole piece or yoke plate 9 is secured to the ends of yoke I by suitablemeans such as resistance welding, shown at II. The plate is providedwith a circular opening l3, and forms with 35 the inner pole piece 5 amagnetic air gap in which a voice coil I5 and form [2, referred to asvoice coil structure, secured to the small end of a frusto-conicaldiaphragm IB is adapted to vibrate in an axial direction. A spacing orcen- 40 tering ring [1, of non-magnetic material, such as brass, issecured to the yoke plate, as by welding or other known method, andserves to accurately space thepole piece 5 and yoke plate opening [3 toform a uniform magnetic gap. 45

The yoke plate is attached to the yoke by means of the interposition ofthe small end 19 of a diaphragm housing or dish-pan adapted to flexiblysupport the base of the diaphragm IS, a portion only of which has beenshown. 5

A field coil 23 is shown around the inner pole for supplying theoperating flux for the magnetic circuit of the speaker. This coilconsists of many turns of small wire through which flows auni-directional current derived ordinarily from 55 a rectifiedalternating current, and containing fluctuations or unfilteredpulsations. This is particularly the case when the field is used as afilter choke in a power supply as disclosed by Rice and Kellogg Patent1,728,879.

During manufacture the cone housing I9, yoke plate 9, and yoke l arefastened together by a single operation resistance welding process:Projeotions which are embossed into the cone housing between adjacentsurfaces of the yoke and yoke plate, melt and solidify in union with thetwo magnet parts as a current and pressure is applied between the yokeplate and yoke. Hum neutralizing elements 2'! and 29 are assembled overthe outside of the core end centering ring H; the field coil 23, core 3and resilient clamping washer 24 are pressed, as a single unit, into theyoke laterally from either open side of the yoke; the core end 5 isdropped into place, being centralized by the centering ring I! to createa uniform air-gap with opening E3; the insertion of the core assemblybolt 7 through a hole in the yoke and through the central bore in thetwo core portions, and locking together with a nut 8, completes themagnet assembly. It will be noted that this process may be easilyreversed Y and the assembly repeated by the service man if occasionarises where the field coil needs to be replaced.

A neutralizing coil 21, of about twenty-five turns, is connected inseries with the voice coil 15 of about fifty turns, on the center core,and the coils are arranged in phase opposing relation for the purpose ofbucking induced hum rising from the fluctuating or unfiltered currentsin the field coil.

A shading ring 29 of proper dimensions and of low resistance material,such as copper, is positioned between the field coil of theelectrodynamic speaker and the neutralizing coil for improving theaction of the neutralizing coil at low frequencies and eliminating highfrequency hum components in accordance with my invention, as disclosedand claimed in application Serial No. 82,579, filed May 29, 1936.

According to my present invention, the voice coil structure is centered,or centrally supported, in the air-gap by providing a smooth slidingcontact surface area between the cell structure and the pole structure.Specifically, the inner cylindrical surface of the coil form [2 istreated by applying an adhesive substance, e. g. shellac, varnish,lacquer, rubber cement or glue, and a soft nap 25 of fibre particles.This may be sprayed on the paper, or other material, constituting thecoil form blank before winding the coil thereon. This nap may be appliedto the entire inner surface of the form or only to sections thereof asmay be illustrated by Fig. 2. Various materials may be used for the nap,e. g., a pile fabric such as velvet, illustrated in Fig. 4. For thepurpose of keeping a small efficient air-gap, it is desirable that thenap be thin, the thickness being exaggerated in the drawing. In factthis invention makes it possible to employ a smaller air-gap than ispossible with the usual spider centering means.

The pole pieces are conventional in design except that the core end orinner pole piece 5 is provided with a smooth glass-like surface that maybe formed by a coating of smooth durable material such as chromiumplating, or a highly polished finish. It is desirable that the core endextend beyond the level of the outer pole piece, or plate 9, and thevoice coil form to provide adequate guiding action during largeexcursions of the voice coil for loud signals. Reference is made to Fig.3 for this arrangement. However, in Fig. 1, the core end is more limitedin length and the edges are rounded off to prevent catching the surfaceof the nap during large movements of the coil.

Referring to Fig. 2 my invention is shown in connection with doublevoice coil structures, a separate guiding structure 25 being shown foreach of the two sections of the coil form that are joined by a flexiblecompliance in accordance with the teachings of Ringel and Olson patentsabove mentioned. Coil structures of this type are longer, heavier andmore difficult of centering support than the ordinary types, and myguiding structure is particularly useful for centering.

Referring to Fig. 3, the outer surface of the coil structure is providedwith a nap sliding means25', forming with the air-gap surface of theouter pole plate a contact area. This may be used instead of the napmeans on the inner surface of the coil structure, if desired, although Ihave shown both in combination here for a particular reason, namely toprevent dust or other foreign particles from entering the air-gap. Dustcaps, outer and inner, of the type heretofore in general use, have beenrather expensive and have added to assembly difficulties. My arrangementnot only performs centering but also dust excluding functions. For thispurpose, a nap in the form of pile fabric such as velvet has particularutility, and Fig. 4 may serve to illustrate this arrangement. Differentmaterial may be used for the nap on the outside of the coil structure,e. g. pile fabric, while using deposited nap on the inside surface ofthe coil structure.

Another advantage in the use of pile fabric is that static friction isreduced since the fibers stand on the pole surfaces on end and flex atthe beginning of a movement. As the movement grows sliding occurs. InFig. 3 the cylindrical inner surface of the outer pole piece is alsohighly polished, or chromium plate, and the edges are preferablyrounded. It is desirable that the nap or fabric be previously treatedwith waterproofing material.

In Fig. 3 the core end or extension 6 (A) is preferably of non-magneticmaterial such as brass. The cylindrical surface of (A) 6 is of chromiumplated or finished same as core end 5.

Outside diameters of (A) 6 and 5 should be the same and the edges of thecontact surfaces should coincide perfectly at the point (B) of contact.By using non-magnetic material for the extension of core 5, the frontleakage magnetic flux is reduced to a minimum and the sensitivity or/andefiiciency of the speaker is not reduced by the extended core.

Although the invention as described above has been shown embodied incertain forms it is to be understood that other equivalent forms arecontemplated and the invention is to be limited only by the scope of theappended claims.

I claim as my invention:

1. In an electromagnetic transducer, pole structure defining an air gapand provided with a polished surface, a sound translating coil structureoperatively disposed for longitudinal vibration in said air gap,centering means having poor sound conduction characteristics secured toa surface area of said coil structure within said air gap and slidablyengaging said polished surface of said pole structure for movement oversaid surface during vibration, whereby a sliding surface contact areaexists only between said means and said" pole structure.

2. The invention as set forth in claim 1 wherein said polished surfacecomprises chromium plating.

3. In a dynamic loudspeaker, inner and outer pole piece structuredefining a magnetic air gap, a voice coil structure operatively disposedin said air gap for longitudinal vibration, a layer of nap materialhaving poor sound conducting characteristics secured to the innersurface of said coil structure and arranged to slidably engage a smoothsurface of said pole structure within said gap during said vibration,for centering said coil structure in said gap, said smooth surface beingformed by a metal plating for reducing friction.

4. In a dynamic loudspeaker, a field structure comprising an innercylindrical removable pole piece and an outer pole piece defining amagnetic air gap, a cylindrical voice coil structure disposed in saidgap for longitudinal vibration, a dia-- phragm attached to said coil,means for centering and guiding said coil comprising a nap surface onsaid coil structure located in said gap, and a polished surface on saidinner pole piece within said gap arranged to form with said nap surfaceduring said vibration a sliding contact surface, said means beingsubstantially a non-conductor of noise from said contact surface to saiddiaphragm.

WILLIAM F. STEERS.

